The present invention generally relates to vibration reduction systems in scanning mirror systems, and relates in particular to reducing vibration and imbalance in scanning mirror systems including limited rotation motors that are used for scanning laser beams over a limited angular range.
Scanning mirror systems include continuously rotating motor systems and limited rotation motor systems. Continuously rotating motor systems for example, may include a rotating polygonal mirror that rotates continuously, and limited rotation motor systems rotate a rotor reciprocally over a limited rotational angular range.
Balancing systems in certain prior art scanning mirror systems involve, in part, correcting for unavoidable imbalances that are caused by parts of the scanning system itself. For example, U.S. Pat. No. 7,316,356 discloses the use of tungsten counterweights for balancing mirrors, wherein the counterweights are disclosed to account for imbalance caused by a precessional torque that is created by rotating a spinner about two axes. The counterweights are not adjustable. U.S. Pat. No. 4,756,586 discloses the use of set screws for adjusting the plane of rotation of a polygonal mirror so that it may be perpendicular to the drive shaft, and further discloses the use of balance screws that are provided to correct an imbalance caused by the set screws. Neither reference discloses finely balancing a mirror mounted on a shaft to account for imperfections in manufacturing.
In typical limited rotation scanner systems, a mirror is mounted to the output shaft of a limited rotation motor (e.g., galvanometer) and the limited rotation motor is controlled by a control loop that seeks to cause the rotor of the motor, and therefore the mirror, to follow a position and velocity command waveform with arbitrarily high fidelity. There are limits, however, on the fidelity with which the system may follow the command.
For example, the acceleration of the mirror within the system is limited by the rate of rise of current in the motor windings. The positional precision is limited by the signal to noise ratio of the feedback method. The bandwidth of the system (which is its ability to move from position A to position B at a desired high velocity and to then settle at position B precisely in the shortest possible time), is limited primarily by vibrations in the moving parts. The bandwidth of the system will nominally be the first torsional resonance in the moving structure. The mirrors therefore, must rotate on the motor shaft back and forth between two angular positions very quickly yet remain very rigid. Not only does this require that the mirrors be made of rigid materials, but their weight must remain low enough that the inertia required to move the mirror from a stop position is not too high.
It is customary, therefore, to make the moving parts as stiff and well balanced as possible within the constraints of the allowable system inertia. Since the torque required of the motor to reach a specified acceleration is directly proportional to the inertia and is proportional to the current (whose rate of rise is limited as noted above), it is often the case that when the system parameters are optimized for a particular inertia, some component, typically the mirror is not as stiff and/or balanced as is required to reach system bandwidth goals, even when made of a very high stiffness-to-inertia material.
Such mirrors for limited rotation motors must therefore be balanced (both front-to-back and side-to-side) so that they introduce as little wobble as possible when moving. In certain applications, extra material is added to the back of the mirror to increase its stiffness and/or balancing, but at the cost of additional inertia, requiring a larger, more expensive motor as well as a control loop that is capable of driving the additional inertia. In some applications for example, such balancing has been done in the industry by adhering small amounts of epoxy onto the back side of the mirror to affect such balancing. Such a procedure however, is not fully satisfactory in all applications such as large-scale manufacturing applications.
Other techniques for balancing mirrors (side-to-side) include the use of trim weights in the form of opposing set screws that are received in opposing bores of a mirror such that any imbalance of the mirror in a plane that includes the cores of the set screws, may be corrected by adjustment of one or both set screws as disclosed in U.S. Pat. No. 6,972,885. The set screws are disclosed to be received in bores in the outer back side and lateral sides of the mirror, but may be difficult to use to achieve a perfect balance when the mirror alone is very closely balanced. Moreover, the use of the set screws as disclosed in U.S. Pat. No. 6,972,885 does not achieve front-to-back balancing, and the set screws add weight (increasing inertia) and may migrate during movement of the mirror.
There is a need therefore, for a limited rotation motor system that provides improved bandwidth without requiring a larger, more expensive motor and accompanying control system, and in particular, there is a need for providing improved mirror balancing without adversely affecting mirror inertia.